As the demand for high performance materials and closely controlled processing parameters increases, there is a growing need for reactor systems that permits the use of ignition sources, such as general purpose electrical devices and high temperature heat sources, near flammable or combustible substances. However, general purpose electrical devices usually have exposed electrical contacts that can contain enough energy to ignite flammable and combustible vapors, while high temperature heat sources can produce surface temperatures that approach and exceed the autoignition temperature of flammable and combustible substances.
Flammable and combustible substances that can be ignited by exposed electrical contact sources are well defined in safety literature. Such substances typically have a low flash point, which is defined by the National Fire Protection Association (NFPA) as the temperature at which a liquid or a volatile solid gives off sufficient vapor to form an ignitable mixture with air near the surface of the liquid or within a test vessel. Flammable or combustible substances also generally have concentration ranges above and below which ignition will not occur. Flammable materials and combustible materials are differentiated by the ease with which they burn.
Flammable materials are those solids, liquids, vapors and gases that ignite easily and burn rapidly. Flammable solids can generally be classified as: (1) dusts or fine powders, (e.g., various metals and organic substances such as cellulose and flour); (2) films, fibers and fabrics constructed of materials having a low ignition point; (3) low temperature autoignition solids such as white phosphorus; and (4) biodegradable solids in which internal heat can be generated by microbial or other degradation activity, (e.g., fish meal and wet cellulosics). Flammable liquids are defined by NFPA and the United States Department of Transportation as liquids having (i) a flash point below 37.7.degree. C. (100.degree. F.), as measured in a closed cup apparatus, and (ii) a vapor pressure of less than 377 kN/m.sup.2 (40 psig). Flammable gases are gases which ignite very easily and sustain a flame and heat propagation rate which is so great as to resemble an explosion, especially when the gas is confined. Flammable gases are extremely dangerous fire hazards and require precisely regulated storage conditions.
A combustible material is generally defined as a solid that is comparatively difficult to ignite and burns relatively slowly, or a liquid having a flash point above 37.7.degree. C. (100.degree. F.). The rate and ease with which solids combust often depends as much on their state of subdivision (e.g., fibrous whiskers versus solid block) as on their chemical composition. For example, many metals will ignite and burn rapidly when in powder or flake form, while most will not burn when formed as a bulk solids. Cellulose is combustible when formed as a textile fabric or paper, but is flammable when provided as a fine fiber such as cotton linters. A plastic that burns at flame temperature is a greater fire hazard when formed as a foam rather than a bulk solid.
NFPA has developed safety standards to permit the safe processing of materials around flammable or combustible substances. These standards are based upon defined classified areas which include (i) Class 1, Group A to D classified areas which involve the presence of flammable vapors, (ii) Class 2, Group E to G classified areas which involve the presence of combustible dust, and (iii) Class 3 classified areas which involve the presence of ignitable figers and flyings. NFPA restricts the use of ignition sources in all of these classified areas. For example, exposed electrical contact sources and unregulated surface temperature heat sources are prohibit in Class 1, Group D areas. Hence, flammable and combustible vapors and gases, as well as materials that emit such vapors and gases, may be stored or processed in areas which include only those types of equipment which have been specially designed for use in such environments.
Equipment suitable for use around flammable and combustible airborne materials is designed to prevent the equipment from serving as an ignition source. Generally, equipment suitable for such uses falls into at least one of five categories: (1) pneumatic or hydraulic equipment, (2) intrinsically safe electrical equipment, (3) explosion proof equipment, (4) remotely located equipment and (5) equipment with regulated surface temperature interlocks. Pneumatic and hydraulic devices use air or fluid to operate and therefore do not serve as ignition sources. Intrinsically safe electrical devices are designed to utilize insufficient electrical current to ignite a flammable or combustible airborne material. Explosion proof devices are retained within shells designed to contain any resultant explosion in order to prevent damage to surrounding objects and people. Remote devices are located outside the area containing the flammable or combustible airborne materials and are attached to a reaction vessel located in the classified area by tubes or insulated wires and connectors. Regulated surface temperature devices are devices whose surface temperature is controlled so that it cannot exceed 80 percent of the lowest autoignition temperature of those flammable and combustible substance which may be present in the area. All five types of these specially designed equipment are typically (i) larger, (ii) more expensive, (iii) less responsive, and/or (iv) produce lower conversion rates, than general purpose electrical devices and unregulated surface temperature heat sources.
Hence, there remains a growing need for a reactor system that permits the use of general purpose electrical devices and/or unregulated surface temperature heat sources on or near reaction vessels that contain or are, of necessity, located in an area containing flammable or combustible substances.